This invention relates to a method for molding a plastic article and, more particularly, to a method for molding a plastic article which includes injecting based on a pressure-dominated control algorithm after detecting an indicia of a decrease in the surface area of the melt front along the leading edge of injected plastic material in order to effectively mold a plastic article without overfilling or packing a mold cavity with plastic material.
Injection molding machines generally include a two-section mold unit wherein a first section is stationary, and a second section is moveable between an open position away from the first section, and a closed position in sealed engagement with the first section in order to form a mold cavity. Typically, a gate opening in the first section allows the injection of plastic material into the mold cavity.
Once the mold cavity has been formed, a reciprocating screw or similar injection device is used to inject a plastic material through the gate opening into the mold cavity where the plastic material hardens with time. As the plastic material is injected into the mold cavity, the leading edge of the plastic material forms a melt front which spreads across the mold cavity as the mold cavity fills. The plastic material is typically injected into the mold cavity very quickly under high pressure to prevent the melt front from hardening too quickly and blocking the mold cavity. Unfortunately, as the mold cavity is filled, the plastic material begins to backflow, causing turbulence and requiring greater clamp pressures and injection pressures.
To overcome the backflow associated with prior art processes, and to assure that the mold cavity becomes completely filled with plastic material, a volume of plastic material greater than the volume of the mold cavity is injected into the mold cavity and maintained under high pressure until the plastic begins to harden. The injection of an excess amount of material into the mold cavity requires a sharp pressure increase in order to pack the material into the mold cavity. This sharp pressure increase is often referred to as a "spike." While injecting excessive plastic material into the mold cavity results in completely filling the mold cavity during the molding process, the resulting pressure spike causes enormous outward pressure on the mold sections, leading to an increased amount of wear on the mold sections and clamping apparatus. Despite constructing mold units of steel to handle the pressure spikes associated with prior art molding processes, these mold units often wear prematurely, requiring the mold units to be returned to the manufacturers for repair. Depending on the severity of the wear and the repair schedule of the manufacturers, worn mold units can be out of service for several weeks. The loss of production associated with the repair of cracked mold units is generally very costly.
The increased outward pressure on the mold sections may even lead to movement of the mold sections away from one another. When the mold sections move apart, plastic material seeps into the parting line between the mold sections. The plastic material which seeps into the parting line and forms as a portion of the finished plastic part is often referred to as "flash." This "flash" is not only aesthetically undesirable on a finished plastic part, but also leads to a waste of material and creates an uneven parting line between the mold sections. As the clamping apparatus presses the mold sections together against the plastic hardening at the parting line, the parting line becomes deformed and uneven, thereby making the occurrence of flash even more likely upon subsequent moldings.
Additional problems associated with prior art high pressure molding techniques include the increased amount of plastic material needed to be placed into the mold cavity, as well as the pressure gradients created throughout the molded part due to the increased amount of plastic material being packed into a finite volume mold cavity. In some cases, the pressure gradient is so large that it leads to warpage of the finished plastic article. If the warpage is great enough, the plastic article may not fit within design tolerances.
While theoretically it would be desirable to fill the mold cavity slowly throughout the molding process, it is difficult to inject the plastic material slowly without uneven curing and blockage resulting within the cavity. If the mold cavity is filled too slowly, the first portion of the plastic material to be injected into the mold cavity begins to harden as the rest of the plastic material is still being injected. This premature hardening leads to clogging of the mold cavity during the filling process. Such clogging is particularly undesirable since it requires stopping a molding run and discarding the partially hardened material.
The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention.